Chemical enhancement is one of the important mechanisms in surface-enhanced Raman spectroscopy, however, its origin is still under debate. Recently, a two dimensional (2D) layered material has been thought to be a strong candidate to investigate the chemical mechanism of Raman enhancement because it has a flat surface, a well defined structure and is without the interference of electromagnetic enhancement. Herein we report systematic studies of the Raman enhancement effect on a gallium selenide (GaSe) flake by using a copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman signal of CuPc on the monolayer GaSe can be significantly increased by one order of magnitude compared to that on a SiO₂/Si substrate. Moreover, the enhancement effect is found to decrease with increasing thickness of the GaSe flake. The origin of the Raman enhancement is attributed to the chemical mechanism resulting from the charge transfer between the GaSe flake and the detected molecules. The supposition is further verified by the investigation of the quenching photoluminescence of GaSe as well as the Raman enhancement effect of CuPc with different thicknesses on the GaSe flake. Our work will shed more light on the understanding of the chemical mechanism for Raman enhancement and expand more practical applications of GaSe.